Equine protozoal myeloencephalitis (EPM) is a debilitating neurologic disease of equines which can affect the brain, the brain stem, spinal cord or any combination of these three areas of the equine's central nervous system. EPM is caused by the protozoan parasites Sarcocystis neurona or Neospora hughesi. 
A horse of any age, breed or gender may be affected by EPM. The disease has been reported in two-month olds, as well as thirty-year olds. In fact, any horse demonstrating neurologic abnormalities may be infected. Clinical signs of a condition depend upon the location of the organism within the central nervous system. These signs include weakness, malposition of a limb, muscle atrophy, spinal ataxia or the like. A severely EPM-affected horse may go down and be unable to rise. Lameness not traceable to orthopedic disease or any combination of the aforementioned signs may occur in early or less severe infections.
Initially EPM was thought to only be caused by Sarcocystis neurona. The opossum (Didelphis virginiana) has been identified as the definitive host for this agents. The intermediate host for this organism is still unknown. The horse ingests feed which has been contaminated with opossum fecal material containing Sarcocystis neurona sporocysts. These sporocysts then excyst in the intestinal epithelium of the intermediate and incidental hosts. In the case of the intermediate host, the merezoites would encyst in the tissues of the host forming sarcocysts. In the case of the aberrant host, the Sarcocystis neurona multiply in the Central Nervous System (spinal cord) and fail to encyst. In horses, the only observed forms of Sarcocystis neurona have been the meront or merozoite.
Recently Neospora hughesi has been identified as a second organism which will cause the EPM clinical disease. Neospora hughesi will not only infect the spinal cord as Sarcocystis neurona does, but will also colonize the brain. At this point in time the definitive and intermediate hosts for Neospora hughesi remain unknown. It is believed that fecal contamination of horse feed or water with sporulated oocysts is the route of horse infection. The oocysts will release tachyzoites which will infect cells as do the merozoites of Sarcocystis neurona. 
In both cases the horse is an aberrant dead-end host and infectious forms of the parasite are not passed from horse to horse or from an infected horse to a definitive or true intermediate host.
There is currently no vaccine or approved animal drug product available for the effective treatment of EPM. The currently available treatments are expensive, of limited efficacy and may include adverse side effects such as anemia, abortion, diarrhea, low white blood cell counts or the like. There remains an unfulfilled need for treatment for EPM-afflicted equines, particularly horses, which is effective, convenient to administer and useful for the reduction of resistant strains.
Therefore, it is an object of this invention to provide an immunogenically active component useful for the prevention or amelioration of EPM.
It is another object of this invention to provide a vaccine composition suitable for use in equines against infection and disease caused by the protozoan parasites Sarcocystis neurona and/or Neospora hughesi. 
It is a further object of this invention to provide a method for the prevention or amelioration of EPM disease in equines that need such protection. Other objects and features of the invention will become apparent from the detailed description set forth herein below.